1. Technical Field
This disclosure relates to an image processing method, a program, a recording medium, an image processing device, and an image forming apparatus.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile, a copying machine, and a composite machine thereof, for example, a liquid ejection type image forming apparatus using a liquid ejecting head (liquid drop ejecting head) as a recording head, for example, an ink jet recording apparatus, is known. The ink jet recording apparatus conducts image formation (wherein recording, character printing, photography printing, and printing may be used as synonyms) by ejecting ink as recording liquid from an ink recording head onto a paper sheet (which is not limited to paper but includes an OHP, means an object to which an ink drop, other liquids, or the like is allowed to adhere, and may be referred to as a medium to be recorded, a recording medium, a recording paper, a recording paper sheet, a recording material, a medium, or the like).
In such an image forming apparatus, recording liquid drops with only about four kinds of sizes (four tones), for example, no dot, a small dot, a middle dot, and a large dot, may be ejected separately, and therefore, a representation with multi-tone is hardly conducted with the dot sizes of the recording liquid drops. Accordingly, a dither method and an error diffusion method are generally known as a method for reproducing halftone by means of a combination of a density tone (intensity tone) and surface area tone (surface area modulation) whose level numbers are less than those of an original one.
The dither method (binary dither method) is a method for determining ‘1’ (image printing or light emission) or “0” (no image printing or no light emission) in comparison with the density of a picture element at a corresponding address point to conduct binary coding while the value of each of dither matrices is a threshold value, and has an advantage such that binary-coded data for a surface area tone may be obtained by only an operation of comparison of original image data with a threshold value and a high speed operation is allowed. Also, the error diffusion method is for diffusing errors provided when tone image data are converted into data with a less level number (for example, binary) onto nearby picture elements so as to reduce the tone representation errors together with the surrounding or nearby picture elements.
In both the dither method and the error diffusion method, the number of levels is three or greater. For example, a multilevel error diffusion method is disclosed in Japanese Patent Application Publication No. 2004-112089.
Also, as a dither matrix used in the dither method, there are known a dispersing type one such as a bayer dither, a random dither and a blue noise dither and a concentration type one in which a sub-matrix is arranged in a dither matrix so as to have a screen angle, as disclosed in Japanese Patent Application Publication No. 2003-259118.
Meanwhile, particularly, in an image forming apparatus with a low precision of ink drop (liquid drop) landing, there is a problem of causing “ejection irregularity” such that an ejected liquid drop is hardly landed on a target landing position thereof by an external disturbance or mechanical factor such as variation of the viscosity of ink (liquid), irregularity of the speed of a carriage mounting a recording head, an error in the precision of manufacture of a nozzle and the like, disturbance of a driving wave pattern supplied to an energy generating device of the head, and the like, whereby a target image or tone characteristic is hardly obtained.
Then, Japanese Patent Application Publication 2005-138494 discloses that timing for supplying a driving frequency to plural recording elements is adjusted to control a landing error caused by ejection irregularity.
However, when timing for supplying a driving frequency to plural recording elements is adjusted to address the liquid drop ejection irregularity caused by the external disturbance or mechanical factor described above, as disclosed in Japanese Patent Application Publication 2005-138494, it is required to modify a processing for each recording element or set the order of liquid drop landing and further to determine a processing depending on an ejection pattern, and therefore, there is a problem such that only an addressing with a particular pattern is allowed.
In particular, since the driving frequency of a deriving wave pattern supplied to a liquid ejecting head tends to increase (attainment of a higher clock frequency) in order to increase a printing speed (decrease a printing time period), the influence of the external disturbance or mechanical factor described above increases so that ejection irregularity of liquid drops is easily caused. The smaller the drop size is, the more the ejection irregularity is easily caused, and it is easily caused in the case where a liquid drop is ejected from a nozzle with a low nozzle precision.